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Because of the nature of this loess material, preconsolidation was deemed necessary in foundation areas which would be subjected to wetting after loading. A geological section at the centerline of the dam is shown on figure 3.
The alluvial materials consist of clays, silts, and fine sand. The material for the most part is poorly graded; the fine and worn fractions exist in separate beds which are often layered and entwined by crossbedding. As studies of the dam foundation progressed, it became obvious that some type of impervious cutoff would have to be incorporated in the dam structure to control seepage.
The Pierre shale, essentially a firm plastic shaley clay, serves as a foundation for the dam and appurtenant structures. It is soft and friable near the surface and becomes harder as the depth increases. Silt, clay, and colloidal material with a small percentage of fine sand are the principal constituents of the shale. It is believed that all joints in the material are watertight. This material is highly impervious and, as it exists in a completely saturated state, high pore pressures develop after application of a load.
5. Cost. - The following tabulation gives the cost of construction of Trenton Dam and Reservoir and the railroad and highway relocations, by major groupings, as of August 28, 1957. A breakdown of these groupings is shown as appendix A, and pay item breakdowns for the two principal specifications, Nos. 2689 and 3047, are shown as appendixes B and C, respectively.